If you use it mainly on travel, I would take the 11’’ iPadPro, with the Keyboard.
It could do what you want to, and is as powerful as an MBP, but could do a lot of things, a MBP could not do…
If you want to use it also at home, I would tend to the 12,9’’ which I have and use every day…
I use my iPad Pro 12.9 extensively for reading and writing. Based on your post, I think the main thing you need to consider is screen size. If you use Split View frequently and/or you need larger text or just more screen real estate for writing, I’d recommend the 12.9 iPad. If screen size is not a major issue, I’d go with the 11” for portability.
I just purchased the 5th Gen M1 iPad Air for my Father’s Birthday and was quite surprised by its performance. My 2018 11" iPad Pro is more than adequate for my needs. If I were to replace it, I would seriously consider the iPad Air.
Based on what your needs seem to be, I think you would be very happy upgrading to the iPad Air.
I bought a MacBook Air M2 recently and can see no reason to replace my IPad 12.9 when it finally gives up. The air is docked to an external monitor when at home and is more than powerful enough for most scenarios
I love my iPad Pro 12.9 inch. It is heavier than the 11 inch, but worth it in my opinion. I can comfortably read PDFs and other documents, using the Apple Pencil to highlight and mark up the documents.